How to set default paper size for scan with continuity camera

How can I set the default paper size when scanning with continuity camera using my iPhone?

I am using the fantastic new Mojave feature to scan documents directly into Preview with my iPhone using File > Import from iPhone > Scan Documents. My paper is a sheet of white A4 paper. The iPhone correctly detects its margins automatically on a darker background and I snap the photo. But when it arrives in Preview I find the image is slightly stretched and measurements (cmd+I) reveal it is 8.5 x 11 letter size.